Pedogenic Carbonates: Forms and Formation Processes

Earth-Science Reviews 157 (2016) 1–17

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Invited review Pedogenic carbonates: Forms and formation processes

Kazem Zamanian a,⁎, Konstantin Pustovoytov b, Yakov Kuzyakov a,c a Department of Soil Science of Temperate Ecosystems, Georg August University of Goettingen, Buesgenweg 2, 37077 Goettingen, Germany b Institute of Soil Science and Land Evaluation (310), University of Hohenheim, Schloss Hohenheim 1, 70599 Stuttgart, Germany c Department of Agricultural Soil Science, University of Goettingen, Buesgenweg 2, 37077 Goettingen, Germany article info abstract

Article history: Soils comprise the largest terrestrial carbon (C) pool, containing both organic and inorganic C. Soil inorganic car- Received 6 September 2015 bon (SIC) was frequently disregarded because (1) it is partly heritage from soil parent material, (2) it undergoes Received in revised form 19 January 2016 slow formation processes and (3) has very slow exchange with atmospheric CO2. The global importance of SIC, Accepted 10 March 2016 however, is reﬂected by the fact that SIC links the long-term geological C cycle with the fast biotic C cycle, and Available online 22 March 2016 this linkage is ongoing in soils. Furthermore, the importance of SIC is at least as high as that of soil organic carbon (SOC) especially in semiarid and arid climates, where SIC comprises the largest C pool. Considering the origin, for- Keywords: Pedogenic carbonate mation processes and morphology, carbonates in soils are categorized into three groups: geogenic carbonates

CaCO3 recrystallization (GC), biogenic carbonates (BC) and pedogenic carbonates (PC). In this review we summarize the available data Diagenesis and theories on forms and formation processes of PC and relate them to environmental factors. After describing Paleoenvironment reconstructions the general formation principles of PC, we present the specific forms and formation processes for PC features and Radiocarbon dating the possibilities to use them to reconstruct soil-forming factors and processes. The following PC are described in Inorganic carbon sequestration detail: earthworm biospheroliths, rhizoliths and calcified roots, hypocoatings, nodules, clast coatings, calcretes and laminar caps. The second part of the review focuses on the isotopic composition of PC: δ13C, Δ14Candδ18O, as well as clumped 13 18 Cand OisotopesknownasΔ47. The isotopic signature of PC enables reconstructing the formation environ- 13 18 14 ment: the dominating vegetation (δ C), temperature (δ OandΔ47), and the age of PC formation (Δ C). The uncertainties in reconstructional and dating studies due to PC recrystallization after formation are discussed and simple approaches to consider recrystallization are summarized. Finally, we suggest the most important future research directions on PC, including the anthropogenic effects of fertilization and soil management. In conclusion, PC are an important part of SIC that reflect the time, periods and formation processes in soils. A mechanistic understanding of PC formation is a prerequisite to predict terrestrial C stocks and changes in the global C cycle, and to link the long-term geological with short-term biological C cycles. © 2016 Elsevier B.V. All rights reserved.

Contents

1. Introduction:inorganiccarboninsoilandpedogeniccarbonates...... 2 1.1. Relevanceofsoilinorganiccarbon...... 2 1.2. Soilinorganiccarbon:worldwidedistribution...... 2 1.3. Soil inorganic carbon pools, classiﬁcation and deﬁnitions...... 2 1.4. Pedogeniccarbonatewithinsoilinorganiccarbonpoolsanditsrelevance...... 3 2. Formationofpedogeniccarbonate...... 3 2.1. Generalprincipleofpedogeniccarbonateformation...... 3 2.2. Formationmechanismsofpedogeniccarbonates...... 4 2.3. Morphologyofpedogeniccarbonates...... 4 2.4. Factorsaffectingpedogeniccarbonateaccumulationinsoil...... 7 2.4.1. Climate...... 8 2.4.2. Soilparentmaterial...... 8 2.4.3. Soilproperties...... 9

⁎ Corresponding author. E-mail address: [email protected] (K. Zamanian).

2.4.4. Topography,soilpositioninthelandscapeandsoilage...... 9 2.4.5. Localvegetationandsoilorganisms...... 9 3. Carbonandoxygeninpedogeniccarbonates...... 10 3.1. Sourcesofcarbon,oxygenandcalciuminpedogeniccarbonates...... 10 3.2. Isotopic composition of carbon (δ13C, Δ14C) and oxygen (δ18O)inpedogeniccarbonates...... 10 4. ImplicationsofPCinpaleoenvironmentalandchronologicalstudies ...... 11 5. Recrystallizationofsoilcarbonates ...... 11 5.1. Uncertaintiesofpaleoenvironmentalreconstructionsbasedonpedogeniccarbonates...... 12 5.2. Evidenceofpedogeniccarbonaterecrystallizationafterformation...... 12 6. Conclusionsandoutlook...... 13 6.1. Conclusions...... 13 6.2. Futureresearchdirections...... 13 Acknowledgements...... 14 References...... 14

1. Introduction: inorganic carbon in soil and pedogenic carbonates other than carbonate-containing minerals, for instance from weathering of igneous rocks, decomposition of organic matter or dissolved Ca2+ in 1.1. Relevance of soil inorganic carbon rainwater (Boettinger and Southard, 1991; Emmerich, 2003; Monger et al., 2015). This calls for investigating SIC stocks, forms and their dy- Soils with 2,470 Pg C (Eswaran et al., 2000) are the largest terrestrial namics to understand the role of SIC in the C cycle at regional and global C pool and are the third greatest C reservoir in the world after oceans scales, the fast and slow processes of C cycling, as well as the link be- with 38,725 Pg (IPCC, 1990) and fossil fuels with 4000 Pg tween biotic and abiotic parts of the C cycle. (Siegenthaler and Sarmiento, 1993) containing organic and inorganic C(Eswaran et al., 2000). Plant litter, rhizodeposits and microbial bio- 1.2. Soil inorganic carbon: worldwide distribution mass are the main sources of the soil organic carbon (SOC) pool. The – – SOC pool comprises 697 Pg C in 0 30 cm and 1500 Pg C in 0 100 cm Large SIC stocks are mostly found in regions with low water avail- depths (IPCC, 2007). Intensive exchange of organic C with the atmo- ability (i.e. arid, semi-arid and sub-humid regions) (Fig. 1)(Eswaran sphere, especially connected with anthropogenic activities, led to a et al., 2000). Low precipitation and high potential evapotranspiration very broad range of studies related to the organic C cycle in soil and strongly limit the dissolution and leaching of carbonates from soil these have been summarized in many reviews (e.g. IPCC, 2007; (Eswaran et al., 2000; Royer, 1999). Accordingly, the highest SIC content Kuzyakov, 2006a). – around320to1280MgCha−1 – is accumulated in soils of arid regions In contrast to organic C, the exchange of soil inorganic carbon (SIC), with mean annual precipitation (MAP) below 250 mm such as middle i.e. various soil carbonate minerals (mostly calcite), with the atmo- east, African Sahara and west USA (Fig. 1). As MAP increases, the SIC sphere and the involvement of SIC in biotic C cycles is much slower content decreases and b40 Mg C ha−1 may accumulate at MAP exceed- (mean residence time of 78,000 years (Schlesinger, 1985)). Additional- ing 1000 mm for example in Amazonian forests and monsoonal forests ly, the distribution depth of SIC is opposite to that of SOC: most stocks in south-east Asia. However, partial eluviation and redistribution of car- ı́ are located deeper than one meter (D az-Hernández et al., 2003; bonates may concentrate SIC deeper in the soil profile (Dıaz-Hernándeź Wang et al., 2010). These two reasons explain why much fewer studies et al., 2003; Wang et al., 2010). focused on SIC than on SOC (Drees et al., 2001; Rawlins et al., 2011). Nonetheless, large stocks of SIC – 160 Pg C in 0–30 cm (Nieder and fi fi Benbi, 2008), 695–748 Pg C in 0–100 cm depth (Batjes, 1996)and 1.3. Soil inorganic carbon pools, classi cation and de nitions 950 Pg C up to 2 m (Lal, 2012) – reflect its importance especially over the long term. The SIC content in first 2 m of soil in semi-arid regions Based on origin, formation processes and morphology, the SIC can be could be 10 or even up to 17 times higher than SOC (Dıaz-Hernándeź subdivided into three large groups: et al., 2003; Emmerich, 2003; Shi et al., 2012). Furthermore, a much lon- 1 ger mean residence time of SIC – millennia (Schlesinger, 1985) – shows 1. Geogenic carbonates (GC) : carbonates which have remained or are its greater role in the global C cycle compared with SOC (few hours to inherited from soil parent materials such as limestone particles or al- centuries) (Hsieh, 1993; Qualls and Bridgham, 2005). SIC also links located onto the soil from other locations by calcareous dust or land- SOC with short residence times to the long-term geological C cycle slides etc. (Liu et al., 2010). Soils of arid and semi-arid regions with usually alka- 2. Biogenic carbonates (BC)1: carbonates formed within terrestrial or line pH (N8.5) and richness in Ca and/or Mg (N0.1%) may enhance the aquatic animals and plants as part of their skeleton, for example SIC content following organic fertilization and increase the respired shells, bones and calcified seeds, or released from or within certain CO2 (Bughio et al., 2016; Wang et al., 2015). organs such as the esophageal glands of earthworms. Changes in environmental properties such as soil acidification be- 3. Pedogenic carbonates (PC): carbonates formed and redistributed in cause of N fertilization, N fixation by legumes or intensification of re- soils via dissolution of the SIC pool (i.e. geogenic, biogenic or previ- wetting cycles due to irrigation could release great amounts of SIC and ously formed pedogenic carbonates) and re-precipitation of dis- increase CO2 emissions (Eswaran et al., 2000; Shi et al., 2012). Such ef- solved ions in various morphologies such as carbonate nodules. fects, though driven by natural processes, are well known in our planet's history, e.g. between the Pleistocene and Holocene, when around 400– This review focuses solely on the origin, morphology and processes 500 Pg C were released from SIC and strongly intensified global of PC formation. The GC and BC are mentioned only if relevant for PC warming over a short period (Adams and Post, 1999). The formation formation. and accumulation of carbonate minerals in soils, in contrast, can directly mitigate the increase of atmospheric CO2 (Landi et al., 2003; Xie et al., 1 Here we do not review the forms and formation of geogenic and biogenic carbonates 2+ 2008) if calcium (Ca ) ions have been released to the soil via sources in soil. K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 3

Fig. 1. World SIC distribution in the top meter of soils (USDA-NRCS, 2000) and its correlation with areas of lower mean annual precipitation. The isolines of mean annual precipitation (mm) are from (FAO, 1996). Only the isolines of precipitation b1000 mm are presented. Note the exponential scale of SIC content. Most SIC is located in areas with precipitation b500 mm and SIC stocks above 32 kg C m−2 (320 Mg C ha−1) are located in areas with precipitation b250 mm.

1.4. Pedogenic carbonate within soil inorganic carbon pools and its uncertainties because of recrystallization and 6) evidence of PC recrys- relevance tallization. Finally, we suggest 7) directions of further studies.

PC originates during soil formation from GC or BC and/or former PC 2. Formation of pedogenic carbonate by recrystallization and redistribution in soil (see Section 2). PC accumulation affects the physical, chemical and biological properties of soil 2.1. General principle of pedogenic carbonate formation (Nordt et al., 1998) and thus affects plant growth and soil productivity. PC accumulation can plug soil pores (Baumhardt and Lascano, 1993; The general process of PC formation consists of three steps: 1) disso- Gile, 1961), increasing bulk density and reducing root penetration, lution of SIC pools,2 2) movement of dissolved ions within pores, water migration and oxygen supply (Baumhardt and Lascano, 1993; throughsoilprofiles as well as landscapes and 3) re-precipitation. Georgen et al., 1991). (1) Dissolution of SIC pools: The dissolution of SIC – mostly of CaCO3 Fine PC crystals (i.e. micrite b4 μm) are more active in chemical re- − − – considering the solubility product (K ≈ 10 6 -10 9)indis- actions than large particles of GC (such as for example limestone). The sp tilled water (Robbins, 1985) is comparatively low (Eq. (1)). The availability of phosphorus and some micro-nutrients such as iron, zinc dissolution rate is strongly controlled by soil pH and dissolved and copper for plants is therefore extremely reduced in the presence CO2. The dissolution rate of CO2 and concentration of dissolved of PC (Becze-Deàk et al., 1997). Accordingly, the presence of PC, their lo- − − inorganic carbon (DIC) species (i.e. HCO ,CO2 ,HCO° and calization and forms in soil modify the water budget and fertilizer 3 3 2 3 CO ), however, is controlled by the partial pressure of CO management. 2 2 (pCO ) in the soil atmosphere (Andrews and Schlesinger, 2001; Considering the effect of PC on plant growth and soil productivity, 2 Karberg et al., 2005). CaCO3 solubility in pure H2Oat25°Cis the layers or horizons containing PC have been defined quantitatively − 0.013 g L 1, whereas in weak acids such as carbonic acid, the sol- (e.g. amounts of carbonate, layer thickness) as diagnostic materials, di- ubility increases up to five times (Aylward, 2007). The acidity agnostic properties or diagnostic horizons in many soil classification − produced by CO dissolution removes OH ions and shifts the systems such as World Reference Base, Soil Taxonomy, Russian and Ger- 2 Eq. (1) to the right, leading to further dissolution of CaCO .The man systems especially in higher levels, i.e. major soil groups (WRB, 3 2014) orders and sub-orders (Soil Survey Staff, 2010). In this review we focus on 1) the general mechanisms of PC formation, 2) the most common morphological forms of PC and their specific formation processes and 3) environmental factors affecting the rate of 2 This is the general formation mechanism of PC. If Ca ions are provided by sources other PC accumulation in soils. We then discuss 4) the importance and appli- than SIC, such as weathering of Ca-bearing silicates, PC may also form (See Section 2.4., cations of PC in environmental sciences and mention 5) the parent material). 4 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17

increase of pCO2 in the soil air increases the solubility of CaCO3, the soil profile (Monger and Adams, 1996; Rabenhorst and otherwise the pH will drop. Wilding, 1986; West et al., 1988). This process commonly redis- tributes carbonates within the soil aggregates and pores of one horizon. (4) Biological models: biological activities increase the concentration þ ↔ 2þ þ − þ ð Þ CaCO3 2H2O Ca ðÞaq 2OH ðÞaq H2CO3aqðÞ 1 of Ca2+ ions inside the organisms (e.g. plant cell-walls, plant vacuoles, fungi hyphae) or close to the organisms (e.g. along roots ‐ þ ↔ ‐ þ 2 þ þ ° þ þ CO2gðÞ H2O H2CO3 HCO3 CO3 CO2aqðÞ H2CO3 ðÞaq 2H due to water mass flow towards the root, below termite nests because of their characteristic residual collectivity). Further calcifi- ð2Þ cation of Ca-bearing organs or supersaturation of soil solution at such sites forms PC (Alonso-Zarza, 1999; Becze-Deàk et al., 1997; Elis, 2002; Monger and Gallegos, 2000; Verrecchia et al., 1995). (2) Movement of dissolved ions: the dissolved Ca2+ ions and DIC species are translocated by water movement in various directions: i.e. diffusion, capillary rise (unidirectional), water percolation (mainly Depending on the prevalence of one or more of these mechanisms downwards) or evaporation (upward). The transportation occurs and their localization, the accumulation of re-precipitated carbonate over multiple spatial scales from mm to km: within and between generates various morphological forms of PC. microaggregates, macroaggregates, soil horizons, landscapes and even from terrestrial to aquatic ecosystems. The dissolved ions, 2.3. Morphology of pedogenic carbonates however, may remain without significant translocation if soil permeability is very low, e.g. at the top of hard bedrock. Despite Around 10 main PC forms are differentiated based on their morphol- downward and upward migration of water, the upward migration ogy, properties and formation mechanisms (Table 1). These PC forms 2+ of Ca ions and DIC species is strongly restricted. Because pCO2 in are classified based on the contribution of biotic and abiotic processes the soil air strongly decreases close to the surface, CaCO3 solubility to their formation as well as PC formation rates. declines, the solution becomes supersaturated and CaCO3 precipitates. The rare cases of upward CaCO3 migration are possible only A) Earthworm biospheroliths: calciferous glands or esophageal from continuously evaporating groundwater (e.g. in calcretes, see glands of earthworms produce carbonate features, which are ex-

Section 2.3.), or in the case of a higher CO2 concentration in the creted in earthworm casts (Fig. 2)(Becze-Deàk et al., 1997). De- topsoil versus subsoil, e.g. due to high microbial and root respira- spite the primary biogenic origin of earthworm biospheroliths, tory activities. they frequently provide an initial nucleus for further spherical (3) Re-precipitation: if soil solution becomes supersaturated with accumulation of other forms of PC. The presence of earthworm

CaCO3, the solutes precipitate. Supersaturation of soil solution in biospheroliths in soils is an indication of stable conditions, i.e. ab- respect to CaCO3 may take place for two reasons: 1) decreasing sence of erosion or deposition (Becze-Deàk et al., 1997). Earth- soil water content mainly connected with evapotranspiration worm biospheroliths occur frequently in loess-paleosol 14 and 2) decreasing pCO2 (Robbins, 1985; Salomons and Mook, sequences (Becze-Deàk et al., 1997)andcanbeusedfor Cdat- 1976). Considering changes in precipitation rates due to environ- ing (Pustovoytov et al., 2004). The formation rate of earthworm mental properties (see Section 2.4) however, various morphol- biospheroliths is fast — within a few days (Lambkin et al., 2011). ogies may form. B) Rhizoliths are formed by mass ﬂow of water with soluble Ca2+

towards the root and precipitation of CaCO3 along the root (Fig. 3 top). Because Ca2+ uptake is much lower than the water 2+ 2.2. Formation mechanisms of pedogenic carbonates uptake, the remaining Ca ions precipitate with CO2 from rhizomicrobial respiration as CaCO3, thus forming the rhizoliths Considering the water movement during PC formation and the mor- (Callot et al., 1982; Hinsinger, 1998; Lambers et al., 2009). The − + other but rare possibility is the release of HCO3 instead of H phology of accumulated PC, various theories and mechanisms have − ﬁ by roots to compensate for the uptake of anions such as NO3 .In- been proposed for PC formation. These can be classi ed into four groups − (adapted from (Monger, 2002)): creasing soil pH by released HCO3 induces CaCO3 precipitation around the root (Klappa, 1980). Rhizolith formation is common (1) Perdescendum models: dissolution of GC, BC or PC in the topsoil, for shrubs and trees, but is not relevant for grasses because of

downward leaching and re-precipitation in subsoil because of their short life cycle. CaCO3 accumulation increases with root water consumption. This is the main process of PC redistribution age over decades to centuries (Gocke et al., 2011a) and may and accumulation in soil horizons (Gile et al., 1966; Machette, form huge rhizolith landscapes, e.g. in Western Australia. In 1985; Royer, 1999). Lateral movement of solutes in this model strongly calcareous soils, plants may reduce Ca2+ toxicity by

also explains PC formation in various positions of a landscape CaCO3 precipitation in vacuoles of root cortical cells. This leads (Monger, 2002). to calcification of the root cortex and formation of another type (2) Perascendum models: PC forms by upward water movement due of rhizoliths termed calcified roots (Jaillard, 1987)(Fig. 3 bot- to capillary rise or fluctuations of shallow groundwater. Dissolu- tom). tion occurs in the subsoil, and upward movement of the solution C) Hypocoatings or pseudomycels are formed by penetration of per-

(after soil dryness because of evaporation, or drop in CO2 concen- colating water through the soil matrix and rapid precipitation of tration) accumulates PC near or even at the soil surface CaCO3 around large and medium soil pores (Fig. 4). Rapid precip- (Khadkikar et al., 1998; Knuteson et al., 1989; Miller et al., itation is common because of the strong pCO2 decrease in these 1987; Monger and Adams, 1996; Suchý, 2002). This model also pores compared to the micro-pores. Hypocoatings may also be includes the dissolution of SIC in higher landscape elevations formed by a ﬂuctuating water table (Durand et al., 2010). Be-

and carbonate migration with groundwater with subsequent cause of fast precipitation, this form of CaCO3 is young, potential- evaporation in soils at lower landscape elevations. ly forming within weeks to months. (3) In situ models: dissolution of SIC pool and re-precipitation of dis- D) Nodules (Fig. 5)areformedin situ by impregnation of soil matrix

solved CaCO3 take place without signiﬁcant movement through with CaCO3 at speciﬁc locations (Durand et al., 2010). This Table 1 Characteristics of the most common pedogenic carbonate features in soils1.

PC features Characteristics Formation5 category Formation time scale

Shape Size Density2 Porosity3 Impurities4

PC features mostly related to biotic controls .Zmna ta./ErhSineRves17(06 1 (2016) 157 Reviews Earth-Science / al. et Zamanian K. Earthworm biospheroliths A Spheroidal Few mm High Moderate Moderate 4 Days Calciﬁed root cells B Branch shape structures Less than mm in diameter and up Low High Low 4 Weeks to months to few cm length Rhizoliths B Cylindrical structures Up to several cm in diameter and High Moderate Low to high outward root center 4 Months to years up to several meters length Needle ﬁber calcite Microscopic needle shape crystals Some μm Very high Very low Very low to pure calcite 4 or could be even Days not pedogenic Pseudomorph calcite after gypsum Microscopic lenticular crystals Some μm Very high Very low Very low to pure calcite Not clear, probably 4 Not clear

PC features mostly related to abiotic controls Soft masses Diffuse powder Visible powder Low High Low probably 3 Weeks Hypocoatings C Laminated carbonate inside soil Few mm thickness with diffuse High Low High 2 Weeks to months matrix and along soil pores boundary into soil matrix Nodules D Spheroidal Few mm to few cm in diameter Low to very high Low to very high High 3 Decades Clast Coatings E Laminated carbonate beneath Few mm to few cm thickness, the High Low Moderate 1, (2)6 Centuries to millennia (or at the top of) clasts same length as related clast Calcretes F Cemented horizon7 At least 10 cm Very high Very low High 2, (1, 3, 4) Millennia Laminar caps G Laminated horizon7 Few mm to even meter Very high Very low Very low 1, (4) Millennia – 17 1 The information for pedogenic carbonate features were inferred considering data in: Alonso-Zarza, 1999; Amundson et al., 1997; Barta, 2011; Becze-Deàk et al., 1997; Brock and Buck, 2005; Candy et al., 2005; Durand et al., 2010; Gile et al., 1966; Gocke et al., 2011a; Khormali et al., 2006; Klappa, 1980; Kovda et al., 2003; Pustovoytov and Leisten, 2002; Rabenhorst and Wilding, 1986; Verrecchia and Verrecchia, 1994; Versteegh et al., 2013; Villagran and Poch, 2014; Wieder and Yaalon, 1982. − 2 Low: 1.5–1.6, moderate: 1.6–1.7, high: 1.7–1.8, very high: 1.8– N 2gcm 3 . 3 Very low: b5, low: 5–20, moderate: 20–30, high: N30%. 4 Very low: b10, low: 10–30, moderate: 30–50, high: N50% (minerals or particles other than calcite). 5 See Section 2.2. (Formation mechanisms of pedogenic carbonate). 6 In parentheses: probable mechanism(s) other than the main one. 7 Calcretes and laminar caps are new soil horizons which are formed by cementation. 5 6 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17

Fig. 2. Earthworm biospheroliths. Left: Plain Polarized Light; PPL (Verrecchia, 2011); Right: Cross Polarized Light, XPL (courtesy O. Ehrmann). Earthworm biospheroliths are produced by −1 −1 earthworms' calciferous glands, which release ~0.8 mg CaCO3 earthworm day (Lambkin et al., 2011). The thin section of biospherolith (right) is kindly provided by Dr. Otto Ehrmann (Bildarchiv Boden, http://www.bildarchiv-boden.de).

impregnation creates the diffuse and gradual outer boundaries of Subsequent desiccation by evaporation or water uptake by

the nodules, and the internal fabric of the nodules remains simi- roots supersaturates the trapped water with CaCO3.CaCO3 then lar to the host soil (Durand et al., 2010). Although nodules are precipitates in microlayers on the bottom of clasts (Fig. 6). The one of the most common forms of PC, the formation processes microlayers usually have light and dark colors, reﬂecting the

and localization of nodules remain unclear. The CaCO3 accumula- presence of impurities. The light-colored microlayers are mostly tion probably initially begins around a nucleus, e.g. mineral par- comprised pure calcite, but the darker one may contain organic

ticles, organic remnants, particles of GC or biospheroliths. compounds and/or minerals other than CaCO3 (Courty et al., Sometimes, nodules have a sharp outer boundary as well as a dis- 1994; Durand et al., 2010). The formation period of coatings is similar fabric as does the host soil (Fig. 5). This probably reﬂects centuries to millennia. Therefore, radiocarbon dating and the sta- soil turbation or translocation of nodules from other horizons or ble isotope composition (δ13Candδ18O) of microlayers represent other parts of the landscape by means of deposition (Kovda et al., an informative chronological and paleoenvironmental proxy 2003). (Fig. 6 left) (Pustovoytov, 2002). E) Coatings on clasts are formed by slowly percolating water be- The formation mechanism of clast coatings, however, is not al- coming trapped on the bottom of clasts such as stone particles. ways similar to that of stalactites. The presence of cracks

Fig. 3. Rhizoliths (top) and calcified roots (bottom). Top left: Rhizoliths formed in loess deposits, Nussloch, south-west Germany (© Zamanian), Top middle and right: Rhizolith formation 2+ stages by soil solution mass flow towards the roots by water uptake (top middle) leading to Ca accumulation and CaCO3 precipitation in the rhizosphere. Root water uptake leads to supersaturation of CaCO3 and precipitation of carbonates, e.g. as calcite along the root. After root death and decomposition of organic tissues the rhizolith remains in soil (top right). Bot- tom left: Calcified roots formed in soils on alluvial deposits (© Zamanian). Bottom right: The magnification of the rectangle on bottom left; note the preserved cell structure and dissolution/re-precipitation in cells. K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 7

Fig. 4. Carbonate hypocoatings. Left: Hypocoatings inside the soil matrix and around the soil pores or cracks (© Kuzyakov), Center: Hypocoating formation by water evaporation or sudden decrease of CO2 partial pressure in large pores, leading to CaCO3 precipitation inside the soil matrix and around large pores. Right: Cross section of PC hypocoating around a pore (XPL) (Courtesy O. Ehrmann). The thin-section of calcite hypocoating around a channel (right) is kindly provided by Dr. Otto Ehrmann (Bildarchiv Boden, http://www.bildarchiv-boden.de).

between the coating and the clast surface creates free space for prolong or shorten the formation period of calcrete (See precipitation of new carbonates (Brock and Buck, 2005). Coat- Section 2.4. Topography and soil position in the landscape and ings may also form at the top of clasts in regions with summer/ soil age). The thickness of calcrete, its location, micromorphology fall precipitation. In wet summers, the stone surface will be and formation stages are useful indicators of development and warmer than the soil solution, leading to supersaturation of bi- age of soils and landscapes (Adamson et al., 2015; Gile et al.,

carbonate on the stone top and consequently CaCO3 precipitation 1966). (Amundson et al., 1997). The alteration in clast coating orienta- G) Laminar caps are formed in the presence of several restrictions tion (i.e. mostly at the bottom of clast), however, is an indicator for vertical water percolation and the subsequent formation of of soil disturbance (Fig. 5 right). a perched water table (Alonso-Zarza, 2003; Gile et al., 1966). Re- F) Calcrete: The soil horizon impregnated and cemented with PC is stricted water permeability leads to lateral water movement at termed calcrete (Goudie, 1972; Reeves, 1970)(Fig. 7). Calcrete the top of the low permeable zone. Such low permeable zones reﬂects the recent or past existence of a shallow groundwater can for example be an existing calcrete or hard bedrock table. Fluctuating groundwater levels accompanied with inten- (Rabenhorst and Wilding, 1986). When the soil becomes dry, sive evapotranspiration accumulate carbonates in soil horizons PC will precipitate in microlayers at the top of the low permeable (Khadkikar et al., 1998; Knuteson et al., 1989), leading to their ce- zone and further decrease the permeability. A laminar cap forms mentation and the formation of calcrete (Fig. 8). Cementation by a new horizon in the soil, which is nearly entirely occupied with 2+ − CaCO3 may occur also by 1) leaching of dissolved Ca and HCO3 PC and is impermeable to roots. Clay minerals and organic matter ions from upper horizons (Fig. 8)(Gile et al., 1966; Machette, comprise non-calcareous materials in this horizon, and soil skel- 1985), or 2) dissolution of Ca2+ containing rock (i.e. limestone) etal particles and coarse fragments such as pebbles and gravels and carbonate precipitation without translocation of dissolved are present in minor amounts and lower than 1% (Fig. 7) ions (Rabenhorst and Wilding, 1986; West et al., 1988). (Brock and Buck, 2009; Gile et al., 1966). The formation of a lam- Biological activities such as bio-mineralization of roots lead to inar cap may also be controlled by biological activity (e.g. the formation of laminar crusts known as rootcretes in soil Cyanobacteria, fungi or horizontal plant roots) (Verrecchia (Verrecchia et al., 1993; Wright et al., 1996). Nonetheless, huge et al., 1995) in the same manner as calcrete formation.

CaCO3 amounts accumulated as calcrete cannot be explained by the translocation of dissolved ions within the soil proﬁle. They clearly reﬂect the Ca2+ relocation from higher landscape positions (Sauer et al., 2015). Considering the formation mecha- 2.4. Factors affecting pedogenic carbonate accumulation in soil nisms, the properties of calcretes, however, will be different: for instance, the presence of high Mg-calcite is an indication of A large complex of several external and internal as well as biotic and groundwater calcrete (Miller et al., 1985). abiotic factors affects the formation processes, accumulation rates and The necessary time span for calcrete formation is millennia or total amounts of PC. The external factors such as climate, topography longer. Soil erosion or deposition may change the depth of max- and organisms mainly affect PC localization and PC formation rates.

imum PC accumulation (Alonso-Zarza, 2003; Gile, 1999)and These factors mainly affect the water balance and CO2 content in the

Fig. 5. Carbonate nodules. Left: PC nodules at lower depths (150 cm) of Voronic Chernosem, “Stone Steppe”, Russia (© Kuzyakov); Right: Cross section of PC nodule and clast coating in the topsoil (A horizon; 0–11 cm) in petric Calcisol (Zamanian, 2005). Photomicrograph is in XPL. 8 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17

Fig. 6. Carbonate coatings on stones. Left: PC accumulation underneath stone particle (i.e. clast coating) and the chronological sequence of microlayers in PC coatings (Pustovoytov et al., 2+ − 2007); Right: Coating formation by percolating water remaining underneath the coarse fragments (e.g. stones). The soluble ions (i.e. Ca and HCO3 ) will precipitate during soil dryness on the bottom side of the stone. In speciﬁc conditions, coatings may form on the upper side of stones (Amundson et al., 1997). The blue arrows show downward migration of water from the soil surface which may partly remain underneath stones. The orange arrows show water evaporation leading to soil dryness and supersaturation of the trapped solution and thus

CaCO3 precipitation.

soil air. The internal soil factors such as parent material and physical and evapotranspiration exceeds precipitation (Birkeland, 1999; Gile et al., chemical properties are mainly responsible for the total amount of PC, 1966; Hough et al., 2014; Rawlins et al., 2011). its morphology and impurities.

2.4.2. Soil parent material

2.4.1. Climate Soil parent material and the Ca source for CaCO3 precipitation affect Climate, i.e. precipitation and temperature, is suggested as the main the total amount, formation rates, mineralogical and isotopic composition controlling factor for PC formation and localization (Borchardt and of PC. There is more PC in soils formed on calcareous parent materials Lienkaemper, 1999; Eswaran et al., 2000). The amount and seasonal dis- (Dıaz-Hernándeź et al., 2003; Schlesinger et al., 1989). Moreover, thicker tribution of mean annual precipitation controls the depth of carbonate PC coatings form under limestone particles (i.e. particles larger than leaching and accumulation (Egli and Fitze, 2001)(seeSection 1.2) 1 cm) compared to sandstones (Pustovoytov, 2002; Treadwell-Steitz (Fig. 9). Therefore, accumulation of PC near the soil surface is common and McFadden, 2000). The source of Ca for PC formation can be examined for precipitation b500 mm (Landi et al., 2003; Retallack, 2005). More- by pursuing trace elements in the PC structure as well as examining the over, MAP controls the soil moisture regime and so, affects the morphol- isotopic composition, e.g. Ca originated from atmospheric deposition is ogy of PC features. For instance, drier conditions may lead to formation evident by similar 87Sr/86Sr ratios in aerosols and accumulated PC 13 of euhedral or well-shaped CaCO3 crystals, whereas anhedral crystals (Chiquet et al., 1999). δ C of PC on calcareous vs. non-calcareous parent with irregular and broken boundaries are formed at more humid pe- materials usually shows a higher heterogeneity i.e. wider range of δ13C, riods (Kuznetsova and Khokhlova, 2012). because GC such as limestone particles remain inside the PC structure The effect of temperature on PC formation, accumulation and locali- (Kraimer and Monger, 2009). In aeolian deposits, however, the ﬁner par- zation is complicated. PC can accumulate in soils in a wide range of tem- ticle size distribution of calcareous dust may lead to complete dissolution peratures from very hot conditions in hot deserts (Amit et al., 2011; of GC and thus less δ13C heterogeneity in PC features (Kraimer and Thomas, 2011) to cold climatic zones such as tundra (Courty et al., Monger, 2009). The weathering of non-calcareous parent materials con-

1994; Pustovoytov, 1998). Increasing temperature decreases CO2 solu- tributes to the localization of cations such as rare earth elements, urani- bility (Krauskopf and Bird, 1994), which directly affects the supersatu- um, barium etc. as impurities in PC structure (Violette et al., 2010; Yang ration of soil solution with CaCO3 (Barker and Cox, 2011). Increasing et al., 2014). temperature, however, boosts microbial respiration and thus increases The weathering of non-calcareous parent materials such as igneous the CO2 concentration in soil air (Lal and Kimble, 2000). This biotic effect rocks in some old soils may provide nearly the total Ca available for PC of temperature overwhelms the abiotic effect of CO2 solubility (Gocke formation (Landi et al., 2003; Naiman et al., 2000; Whipkey et al., and Kuzyakov, 2011). Accordingly, higher temperatures increase the 2000). However, it usually supplies b2% of Ca in precipitated PC (Capo PC accumulation rates (Candy and Black, 2009; Gocke and Kuzyakov, and Chadwick, 1999). The presence of co-precipitated cations from par- 2011). Faster rates (due to warmer conditions) lead to more impurities ent material in the PC structure changes the crystallographic parameters such as rare earth elements (REE) in the PC structure (Gabitov et al., of CaCO3 and controls the crystal morphology (Klein, 2002). For in- 2008; Violette et al., 2010). The presence of such co-precipitates affects stance, impurities decrease the crystal size (Catoni et al., 2012). Elongat- the dissolution rate of PC after formation as well as its morphology and ed and needle-shaped crystals are formed in solution at higher crystal size (Eisenlohr et al., 1999) (see Section 4.3.2). concentrations (100 ppb) of (REE3+)⁄(Ca2+), while rhombohedral The temperature controls PC morphology and the formation of and prismatic crystals are common at lower concentrations (10 ppb)

CaCO3 polymorphs (Ma et al., 2010). Increase of temperature decreases (Barker and Cox, 2011). The impurities also inhibit PC dissolution be- 2− 2+ the [CO3 ]/[Ca ] ratio and so, aragonite formation is favored instead of cause they remain on the crystal surface and decrease ion exchange calcite or vaterite (Ma et al., 2010). (Eisenlohr et al., 1999). In conclusion, the balance between MAP (total amount and season- Aluminosilicates as well as organic compounds such as fulvic and ality) and evapotranspiration (driven by temperature and wind speed) humic acids are additional impurities (Gabitov et al., 2008; Stumm determines the rates and the amounts of PC as well as the depth of PC and Morgan, 1996). The presence of aluminosilicates and organic com- accumulation. Hence PC are formed during soil drying when pounds in PC structure affects crystal growth. For instance, binding of K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 9

2009; Ma et al., 2010; Reddy, 2012). Soil properties such as texture and structure control the accumulation depth of PC because they affect water holding capacity, water penetration and movement (Chadwick et al., 1989). The pH affects carbonate crystal size and morphology by

controlling the supersaturation state of soil solution with CaCO3 (Ma et al., 2010). The ratio of bicarbonate/carbonate decreases as the soil pH becomes alkaline (e.g. pH N 8.5). This favors higher nucleation

rates and faster precipitation of smaller CaCO3 crystals (Ma et al., 2010). Ionic strength controls the mole fraction of free water during

CaCO3 dissolution (Finneran and Morse, 2009). Therefore, CaCO3 dissolution in saline soils takes longer and precipitation occurs earlier compared to salt-free soils.

2.4.4. Topography, soil position in the landscape and soil age The topography and soil position in a landscape affect the total amounts, the accumulation rate and the accumulation depth of PC. The upper parts of a hillslope may contain no or few PC features, while thick calcretes may form at downslope positions because of groundwater presence or downslope flow of soil solution (Jacks and Sharma, 1995; Khadkikar et al., 1998). Stable land surfaces in a landscape usually show the greatest PC accumulation compared to the other positions. On unstable land surfaces the depth of PC accumulation and the total amount of PC in soil changes due to erosion and deposition. Erosion increases the PC exposure into the percolating water front, and rewetting cycles promotes carbonate dissolution. PC dissolution followed by the translocation of ions leads to less PC accumulation in the soil profile or their deeper localization. It can lead to complex profiles with overprinting over multiple formation phases that have been formed during various climate cycles. Deposition also changes the depth of water percolation, reducing the PC accumulation in a particular depth of the soil profile (Alonso-Zarza, 2003; Candy and Black, 2009; Gile, 1999). On stable land surfaces, total PC is positively correlated with soil age. Increasing amounts over time also creates various PC morphologies (Adamson et al., 2015; Badía et al., 2009; Bockheim and Douglass, 2006; Dı́az-Hernández et al., 2003). Disperse PC accumulations increase with soil age, will be connected to each other and finally plug soil pores, forming calcrete (Fig.8). Therefore, various morphologies and stages for PC accumulation are used as an indicator of soil development (Fig. 8)(Amoroso, 2006; Gile et al., 1966; Machette, 1985; Pustovoytov, 2003).

2.4.5. Local vegetation and soil organisms In the presence of active roots, carbonate dissolution increases by 5 to 10 times. Carbonate solubility increases near roots because of

(1) up to 100 times higher CO2 concentration in the rhizosphere versus atmosphere and (2) up to two units lower local pH because of H+ and carboxylic acid release by roots (Andrews and Schlesinger, 2001; Berthelin, 1988; Gocke et al., 2011b). The higher ions concentration leads to two-orders-of-magnitude-faster PC accumulation close to the roots compared to root-free soil (Gocke et al., 2011b; Kuzyakov et al., 2006), e.g. to rhizolith formation (Fig. 3). Note, however, differences in root distribution and thickness as well as variation in root respiration and exudation (Hamada and Tanaka, 2001; Kuzyakov and Domanski, Fig. 7. Calcrete morphology. Top: thick calcrete formed on alluvial deposits comprised two 2002) change the PC formation rates under various plant species. For distinct horizons: the lower calcrete contains abundant coarse fragments impregnated example, carbonate dissolution and re-precipitation under maize is – — and cemented with PC. The upper calcrete laminar calcrete comprises negligible higher than in soils covered by ryegrass because the root growing coarse fragments but horizontal layers of PC accumulation (proﬁle depth: ca. 150 cm). Middle: PC accumulation as microlayers in the upper calcrete. Bottom: Surrounded rates and exudation are higher under maize. coarse fragments with micritic PC in the lower calcrete. Photomicrographs are in XPL Soil microorganisms, i.e. bacteria and fungi, are also active in PC for- (Zamanian, 2005). mation. If Ca2+ ions are available in solution, bacteria can produce a visible accumulation of carbonates within a few days (Monger et al., 1991). carboxyl groups at or near crystal growth sites inhibits the growth rate Extracellular polymers such as polysaccharides and amino acids may of CaCO3 crystals (Reddy, 2012). also control the morphology of CaCO3 (Braissant et al., 2003). For example the presence of aspartic acids favors the formation of needle shape 2.4.3. Soil properties crystals (Braissant et al., 2003). However, even components of bacterial Soil texture, structure, pH, ion strength and composition of soil solu- cells such as cell walls may act as nuclei of carbonate precipitation tion can affect PC formation (Chadwick et al., 1989; Finneran and Morse, (Perito et al., 2014). 10 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17

Fig. 8. Calcrete formation: Accumulation of PC by CaCO3 redistribution within a landscape: CaCO3 will be mainly leached from upper parts of the landscape with groundwater (inclined blue arrows) and will be moved to a lower landscape positions. Upward movement of water by capillary rise (vertical blue arrow) will form calcrete at the middle parts of a landscape.

CaCO3 relocated from higher landscape positions cements the carbonate deposition zone (calcic horizon) and ﬁnally form the calcrete (Knuteson et al., 1989). Considering the four steps of Gile et al.’s (1966) model, formation of coatings on clasts is the initial stages of PC accumulation in gravelly soils (right), while in non-gravelly soils (left) nodules would be formed. Connection of coatings as well as nodules by the gradual CaCO3 accumulation will plug the soil horizon (stage III) and forms calcrete. Water stagnation at the top of calcrete and subsequent gently drying soil will generate a laminar cap at the top of calcrete in the same way in gravelly and non-gravelly soils (stage IV).

3. Carbon and oxygen in pedogenic carbonates litter and SOM are the sole CO2 sources in the soil air during the growing season (Karberg et al., 2005). However, in frozen soils or soils with very

3.1. Sources of carbon, oxygen and calcium in pedogenic carbonates low respiration rates (e.g. dry hot deserts), the CO2 concentration is partly controlled by the diffusion of atmospheric CO2 into the soil Carbon in PC originates from dissolved CO2 in soil solution (Eqs. (1), (Cerling, 1984). (2)). Respiration of roots and microorganisms and the decomposition of The source of oxygen in PC is related more to the soil water than to 18 soil CO2.Thisisconﬁrmed by the close correlation between δ OofPC and mean δ18O of local meteoric water (Cerling, 1984; Cerling and Quade, 1993). Calcium in PC can originate from three sources: (1) dissolution of GC as limestone (and/or to a lesser extent dolostone) (Kelly et al., 1991; Rabenhorst and Wilding, 1986), (2) atmospheric deposition, which is the main source of Ca especially in non-calcareous soils (Naiman et al., 2000) and (3) weathering of Ca-bearing minerals other than carbonates (Landi et al., 2003; Naiman et al., 2000; Whipkey et al., 2000)suchasau- gite, apatite, hornblende, gypsum, oligoclase and plagioclase.

3.2. Isotopic composition of carbon (δ13C, Δ14C) and oxygen (δ18O) in pedogenic carbonates

The isotopic signature of PC – δ13Candδ18O – is controlled by the iso-

topic composition of soil CO2 and of water, respectively (Cerling, 1984). During the growing season, root and microorganism respiration is high

and represents the only CO2 source in soil (Cerling, 1984); the relative 13 abundance of C3 and C4 plants in the local vegetation controls the δ C value of PC (Fig. 10). Due to isotopic discrimination by photosynthetic 13 pathways, the δ CofCO2 under C3 plant species (−27‰ on average) differs from that under C4 species (−13‰ on average) (Cerling et al., 1997). Further isotopic discrimination results from CO2 diffusion in soil (ca. +4.4‰) and carbonate precipitation (ca. +11‰). Consequent- 13 ly, PC are Cenrichedbyabout15‰ compared to the respired CO2.The values are ca. -12‰ under pure C3 and +2‰ under pure C4 vegetation (Fig. 10).

Since root and rhizomicrobial respiration are the dominant CO2 sources in soils (Kuzyakov, 2006a), SOM decomposition has a minor effect on 13CofPC(Ueda et al., 2005). Diffusion of atmospheric CO (global Fig. 9. Correlation between the mean annual precipitation (MAP) and the upper depth of the 2 δ13 − ‰ 13 pedogenic carbonate horizon (Bk) (data in Heidari et al., 2004; Khormali et al., 2012, 2006, average C= 8.5 in 2015) can further enrich C in PC. Nonethe- 2003; Khresat, 2001; Kovda et al., 2014; Kuzyakov, 2006b; Royer, 1999, n = 1542). less, the effect of diffused atmospheric CO2 is restricted maximally to the K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 11

Pustovoytov et al., 2007a, 2007b; Pustovoytov and Leisten, 2002; Wang et al., 1996). The radiocarbon ages help to distinguish between individual stages of PC formation and correlate them to past environmental changes (Fig. 6)(Candy and Black, 2009; Pustovoytov et al., 2007a, 2007b). Along with radiocarbon dating (age limit up to 60,000 years), the Th/U-technique allows estimation of crystal growth within longer time intervals during soil formation (age determination up to over 500,000 y) (Ku et al., 1979; Sharp et al., 2003; Candy et al., 2005; Durand et al., 2007; Blisniuk et al., 2012). Uranium may be incorporated into the PC structure as impurities during crystal growth (see Section 3.3, parent material). The broader age range that can be determined is the major advantage of application of the Th/U-dating to Qua- ternary carbonate materials. Some sedimentological settings have been suggested to be favorable for diagenetic contamination of carbonate by environmental uranium, which may result in younger measured ages (McLaren and Rowe, 1996). However, the Th/U ages of different carbonate samples usually show a good match with independently esti- Fig. 10. The δ13C values of carbonate forms in soil (changed after Nordt et al., 1996). The mated ages of their contexts. Such inter comparisons are based on 13 δ C isotopic composition of soil CO2 and thus of pedogenic carbonates (PC) is archeological age estimations (Magnani et al., 2007), OSL and radiocar- δ13 controlled by local vegetation (C3 or C4 plants) (Cerling, 1984). The mean C value and bon dating (Magee et al., 2009) or their combinations (Clark-Balzan standard deviation for biogenic carbonates (BC) are calculated from: Dettman et al., et al., 2012). Although the sample quantities required for Th/U dating 1999; Prendergast et al., 2015; Pustovoytov et al., 2010; Regev et al., 2011; Riera et al., 14 13 are larger compared to the C AMS procedure, substantial reduction 2013; Stern et al., 1994. Note the different C fractionation by rhizorespiration for C3 13 and C4 plants (Werth and Kuzyakov, 2010). The C fractionations are presented with in sample size can be achieved through the use of multi-collector induc- dashed lines and mentioned in italics. tively coupled plasma mass spectrometry (Seth et al., 2003) and laser ablation techniques (Spooner et al., 2016). 13 upper 50 cm of the soil (Cerling, 1984) and is negligible in the presence Since the δ CofPCreflects that of soil CO2 and is related to the pCO2 13 of vegetation. in soil air and in the atmosphere, the δ C of PC can be used as a CO2 Δ14 The C of PC is determined by biological activities in soil. In con- paleobarometer to estimate the atmospheric CO2 concentration during trast to δ13C, SOM decomposition affects Δ14C in PC. Therefore, the rela- the formation time of PC (Huang et al., 2012; Retallack, 2009). This tive proportion of CO2 respired by the rhizosphere and the CO2 released CO2 paleobarometer shows a high potential for paleosols covered with 14 from SOM decomposition determine the C abundance in PC. The con- pure C3 vegetation (presumably most pre-Miocene soils) or if the pro- 14 tribution of SOM decomposition to C abundance in PC, however, is portion of C4 biomass can be estimated (for example, if the humus hori- more important in deeper horizons. This is because the SOM age mostly zons are preserved) (Ekart et al., 1999; Royer et al., 2001). increases with soil depth (i.e. the older the SOM, the more depleted the The δ13Candδ18O in the lattice of PC crystals enable estimating the 14C abundance) (Amundson et al., 1994). temperature of PC formation (Ghosh et al., 2006a). The combination of δ18 13 18 The O of PC is controlled by the oxygen isotopic composition of Cand O in CaCO3 crystals, known as Δ47 or clumped isotopes, is meteoric water, from which carbonates originate (Cerling, 1984). In- the measuring δ18Oandδ13C connected in one molecule simultaneously, δ18 13 18 16 creasing evaporation leads to higher O depletion in PC (Liuetal., for instance as C O O. The Δ47 value in a crystal lattice depends only 1996; Zhou and Chafetz, 2010). Since the temperature controls the on the environmental temperature: increasing the temperature will de- amount of evaporation, changes in the isotopic composition of meteoric crease the Δ47 in that crystal (Eiler, 2007). Therefore, the Δ47 value in PC water corresponds to mean annual air temperature (MAAT) (Cerling, can be used as a paleothermometer to estimate the temperature during 1984; Hsieh et al., 1998a, 1998b). PC formation (Ghosh et al., 2006a; Versteegh et al., 2013). The estimated PC formation temperature and the relation between environmental 4. Implications of PC in paleoenvironmental and chronological temperature and elevation enable drawing conclusions about the uplift studies range of geological surfaces (Ghosh et al., 2006b). Accordingly, the PC features now located at higher elevations with cooler temperature δ13Candδ18OaswellasΔ14C of PC are valuable proxies for may have been formed in warmer environments (Peters et al., 2013). paleoenvironmental and chronological investigations (Feakins et al., 2013; Levin et al., 2011; Monger et al., 2009; Pustovoytov et al., 2007a, 5. Recrystallization of soil carbonates 2007b; Wang et al., 1996). Dissolution of SIC and re-precipitation of dissolved ions (i.e. Ca2+ and DIC species) takes place under complete equi- All the above-mentioned applications of δ13C, Δ14C, δ18Oand librium with soil air CO2 (Eq. 3)(Cerling, 1984; Nordt et al., 1996). clumped isotopes in PC are based on two assumptions: (1) The formed PC feature is completely free of GC admixtures. þ G R G R ‐ CaC O3 þ C O2 þ H2O↔Ca HC O3 þ HC O3 ↔ (2) The formed PC feature represents a geochemically closed system. þ ð3Þ This means PC experiences no further cycle(s) of dissolution and re- ↔ R þ G ‐↔ R ↓ þ G þ Ca HC O3 HC O3 CaC O3 C O2 H2O precipitation (= recrystallization) after initial formation. Deviations from these assumptions reveal uncertainties in chrono- where the index G reflects the origin of carbon from geogenic car- logical and re-constructional studies based on PC (Cerling, 1991; bonate present in soil before dissolution and R reflects the carbon origin Pustovoytov and Leisten, 2002; Quast et al., 2006). Because recrystalli- from CO2 respired by roots and microorganisms. Therefore, substituting zation rates depend on various biotic and abiotic factors (Gocke et al., G − R − 13 HC O3 by HC O3 will conserve the δ C fingerprints of dominant veg- 2011b; Gocke and Kuzyakov, 2011), the resulting errors will differ, es- etation within accumulated PC (Fig. 10)(Amundson et al., 1989; Cerling pecially where recrystallization is relatively fast, e.g. in the presence of et al., 1989). high root and microbial respiration (Gocke et al., 2011b; Kuzyakov The Δ14C of PC is applied to determine the absolute age of soils, sedi- et al., 2006). −6 −9 ments, cultural layers and late-Quaternary geomorphological units The low solubility of carbonates (Ksp =10 -10 )(Robbins, (Amundson et al., 1994; Chen and Polach, 1986; Gile, 1993; 1985) and consequently low recrystallization rates lead to difficulties 12 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 in measuring these rates over short periods. Recently, however, it has • Old wood ashes (Regev et al., 2011) and calcified fossil seeds been shown that the sensitive 14C labeling approach (Gocke et al., (Pustovoytov et al., 2004; Regev et al., 2011). 2011b, Gocke et al., 2010; Kuzyakov et al., 2006) can contribute to a bet- ter understanding of the recrystallization dynamics and their effects on the isotopic composition of C in PC. This technique labels soil air with BC features are used to recognize the settlements or habitats, diet re- 14 14 gimes and extinction periods of ancient humans, animals and plants CO2. By tracing the C activity of a carbonate sample and knowing (Hoppe et al., 2004; Janz et al., 2009; Kandel and Conard, 2005)as the amounts of added C as CO2, the amounts of recrystallized carbonates can be calculated. This approach was used to show the dependence of well as to reconstruct the environmental conditions during their life- times (Villagran and Poch, 2014; Xu et al., 2010; Yanes et al., 2013). CaCO3 recrystallization rates on (i) CO2 concentration in soil (Gocke et al., 2010), (ii) presence of plants with various root systems (Gocke PC formation and mixing with fossil BC will complicate the results of et al., 2011b), (iii) temperature (Gocke and Kuzyakov, 2011), and (iv) such paleo-reconstruction studies, e.g. the age of a 45,000 y-old bone will be estimated 20,000 y if only 5% contamination with modern C migration of recrystallized CaCO3 along soil profile (Gocke et al., 2012). The very slow rates assessed by the 14Clabelingapproach took place (Zazzo and Saliège, 2011). Paleoenvironmental reconstruc- (about 0.00003 day−1) demonstrated that at least centuries or probably tions and dating based on PC as well as BC should consider possible re- even several millennia are necessary for full recrystallization and thus crystallization and isotopic exchange. for complete formation of PC (Kuzyakov et al., 2006). This means that the first assumption may not be achieve even after a long time, at 5.2. Evidence of pedogenic carbonate recrystallization after formation least in PC features formed in loess deposits. Furthermore, the exponential nature of recrystallization (Kuzyakov et al., 2006) – partial re- The recrystallization of PC features after formation can be recognized dissolution and recrystallization of formed PC – may also make the in isotopic composition as well as morphology. The following evidence fi second assumption questionable. con rms the recrystallization of PC features in different environmental conditions as the dominant process during their formation.

(a) Relatively young radiocarbon ages of PC features compared to 5.1. Uncertainties of paleoenvironmental reconstructions based on geological periods are usually explained by admixtures of mod- pedogenic carbonates ern 14C during recrystallization (Pustovoytov and Terhorst, 2004). A correspondence between measured Δ14CagesofPC The recrystallization of PC under conditions different from the envi- with other chronological data is therefore used to evaluate the ronment during PC formation (e.g. changes in local vegetation or envi- PC contamination and the reliability of achieved dates. The ronmental temperature) will strongly complicate the application of other chronological data include stratigraphy of the sampling the isotopic signature of PC for paleo-reconstruction studies. The new context or the ages of accompanying datable compounds such fl isotopic signals of a PC feature will re ect the altered and not the origi- as organic C and artefacts (Pustovoytov and Terhorst, 2004; fi nal environmental conditions (Pendall et al., 1994). Considering the rst Vogel et al., 2001). “ ” assumption, mixing of old PC as well as dead (i.e. not applicable for ra- (b) Large δ13C variation in PC from paleosols with similar ages (and diocarbon dating) limestone particles with newly formed PC overesti- probable similar vegetation and pCO2 in the respective geological mates the absolute ages of soils, landscape or geomorphological period) is referred to recrystallization. In contrast, fewer δ13C dif- surfaces (Pendall et al., 1994; Pustovoytov and Terhorst, 2004). For in- ferences in PC from contrasting geological time spans are also instance, if only 1% dead limestone particles remain in the structure of a troduced as recrystallization evidence (Quast et al., 2006). given PC specimen (i.e. not full recrystallization of GC), the age of PC (c) The size of PC features is positively correlated to the δ13C signa- will be overestimated by more than two times. If the amount of remain- ture of recently recrystallized carbonates (Kraimer and Monger, ing GC is 5%, the age overestimation will increase to about 10 times 2009). The smaller the PC size, the more δ13C changes due to re- δ13 (Kuzyakov et al., 2006). Moreover, the difference of C values of the crystallization is expected. δ13 remaining GC to that of PC (Fig. 10) leads to a less negative CofPC (d) The microscopic indications of PC dissolution under a polarized (Pendall et al., 1994; Quast et al., 2006) and consequently to doubtful microscope can be recognized as follows (Durand et al., 2010): paleoecological interpretations. Recrystallization will also affect the stable isotopic signature and interpretations for paleoenvironmental studies and PC-based radiocarbon 1. PC grains with well-rounded shapes. dating. If only 1% of modern 14C is mixed with a dead limestone speci- 2. Presence of crystals with pronounced serration. men, the age estimation will be 36,500 years. Increasing the contamina- 3. Formation of mouldic voids (e.g. preferential dissolution of shell frag- tion with modern 14C to 10% alters the age of that limestone to about ments leaves empty spaces previously occupied by carbonates). 18,500 years (Williams and Polach, 1969). 4. Clay-coating networks without carbonate crystals (formed after par- PC recrystallization also affects δ18O(Cerling, 1991) and may thus tial dissolution of carbonate grains and further clay illuviation with overestimate PC formation temperature by up to 20 °C (Ghosh et al., pore filling). 2006b). Therefore, interpretation of the δ13C, Δ14C, δ18OandΔ signa- 47 5. Depletion of hypocoatings (i.e. soil carbonate-free matrix around tures in PC for paleoenvironmental reconstructions and dating should voids such as channels). consider possible deviations from the above-mentioned assumptions. Formation of PC following BC dissolution will also affect the chronological and paleoecological interpretations based on BC. In archeological sites, various BC types preserved in soils are frequently used to interpret (e) The dissolved ions may recrystallize on the former PC feature. their isotopic signatures. This includes: The microscopic evidence of such recrystallization is (Durand et al., 2010): • Shells (i.e. mollusk shells) (Xu et al., 2010; Yanes et al., 2013) • Bone pieces (Berna et al., 2004; Krueger, 1991; Zazzo et al., 2009) • Eggshell particles (Janz et al., 2009; Kandel and Conard, 2005; Long 1. Irregular distribution of crystal size and mottled crystal mosaics of et al., 1983; Vogel et al., 2001) different sizes (i.e. replacement of finer crystals with coarser ones). • Tooth enamel and dentin (Feakins et al., 2013; Hedges et al., 1995; 2. Star-like masses of elongated and radially arranged sparite crystals Hoppe et al., 2004) around a central zone of microsparite crystals. K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 13

3. Curved contacts between neighboring sparitic (N20 μm) carbonate as comparisons of the Ca content in parent material as well crystals. as in soil layers with PC are needed to identify the Ca source(s) in PC. 6. Conclusions and outlook (2) Implications for paleoenvironment reconstructions and soil 6.1. Conclusions genesis. − The reliability of PC features as proxies for paleoenvironment re- Various formation mechanisms and environmental factors result in constructions and dating purposes is still questionable because distinct morphological features of PC such as nodules and coatings, of recrystallization. This calls for quantifying how the environ- — which form in various time spans from a few weeks (e.g. hypocoatings) mental factors such as soil moisture, temperature, initial GC con- and decades (e.g. rhizoliths) to hundreds of thousands or even millions of tent, and the depth of PC formation affect PC recrystallization. In fl years (e.g. calcrete). PC forms therefore re ect soil genesis processes and this respect, 14C labeling of soil CO showed high potential for un- δ13 Δ14 δ18 2 record the effects of soil-forming factors. C, Cand Oaswellas derstanding the dynamics of carbonate recrystallization in soils Δ 47 in PC are valuable tools for paleoenvironmental reconstructions and (Kuzyakov et al., 2006). The radiometric ages of PC features should soil age estimation. PC features, however, have variable physical and be compared with independently estimated ages of their contexts, chemical properties including various CaCO3 contents and impurities. such as archeological sites or geomorphic landscape elements. This reflects the response of PC features to environmental conditions Furthermore, long-term experimental observation of CaCO3 alter- such as changes in local vegetation or climatic properties. Furthermore, ation with time in native soils can serve as a good complimentary depending on the duration of PC formation period, the isotopic inventory approach. of individual PC features will reveal different resolutions in paleo- − Individual PC features, considering variations in their physical and reconstruction and chronological studies. chemical properties, should respond differently to changes in en- PC can undergo recrystallization after formation. This complicates vironmental conditions, i.e. will have different recrystallization the interpretations of paleoenvironment records and chronological rates. Therefore, the recrystallization rates of various PC features studies based on PC isotopic composition. Every recrystallization cycle should be compared under identical environmental conditions. may occur under new environmental conditions – i.e. climate or local 13 − A part of C enrichment in PC comparing to the respired CO2 is be- vegetation – differing from the previous one. Full or even partial re- cause of soil CO2 diffusion (Cerling, 1984). The CO2 diffusion in soil equilibration to the new environment will insert new signals into the (and thus changes in δ13C of PC) is, however, related to soil prop- isotopic inventory of PC. Recrystallization therefore resets the radiomet- erties such as soil water content, temperature and clay content as 14 ric clock by adding modern C to the isotopic inventory of PC. It can well as the diffusion distance within the soil profile. The above- therefore lead to a strongly biased assessment of air pCO2 or tempera- 13 mentioned 4.4‰ C enrichment in soil CO2 by diffusion should ture (as well as vegetation or precipitation) for the period of PC forma- therefore be analyzed for various soils with contrasting physical tion. The result is misleading paleoenvironmental reconstructions. and chemical properties. Nonetheless, incorporating the variety of PC features (with corresponding formation mechanisms and time, as well as physical and chemical (3) Natural and anthropogenic effects on PC and consequences for properties and microscopic indications) enables considering how re- the concentration of atmospheric CO . crystallization may have altered the isotopic composition of PC features. 2 − The contribution of CaCO3 to CO2 fluxes from soil to the atmo- 6.2. Future research directions sphere because of fertilization and management is completely un- known. Soil acidification due to urea or ammonium fertilization as

Based on the overview of the mechanisms and rates of PC formation well as legume cultivation strongly affects CaCO3 dissolution and and of their applications for reconstructing soil genesis and CO2 release to the atmosphere. This calls for investigating the ef- paleoenvironment, as well as considering the huge SIC stocks in soil, fects of various soil cultivation systems such as fertilizer forms the following research directions can be grouped into three issues: and levels, as well as management practices — till, no-till, liming, irrigation frequency and other managements — on CaCO3 dissolu- (1) Mechanisms and rates of PC formation. tion and CO2 efflux. These anthropogenic effects on CaCO3 dissolu- − The effects of biotic processes such as respiration (CO2 concentration should be compared to the rates of natural acidification tion), carboxylic acid excretion (pH changes) or water uptake (Ca processes related to litter decomposition and rhizosphere fluxes concentration in rhizosphere) by plants and microorganisms on of H+ ions and organic acids. PC formation were shown in a few studies (Kuzyakov et al., − Development of a mechanism-based model predicting the upper 2006; Monger et al., 1991). However, the biotic activities are fre- and maximal depths of PC accumulation in soil profiles is impor- quently disregarded with respect to PC formation. This calls for tant for understanding soil genesis as well as fertilization and irri- demonstrating the importance of biota for PC formation under a gation management. This requires incorporating the relations broad range of environmental conditions. It remains unclear between the depth of PC accumulation and various environmental whether PC can be formed in the absence of biological activity at parameters — not only mean annual precipitation as in Fig. 9 but all. also soil water balance, its seasonal dynamics, the initial carbonate − Both roots and microorganisms may have similar functions in PC content in parent material and soil physical properties. formation: respiration, acid release, etc. We are not aware of any study comparing the importance of roots or microorganisms for PC formation. This should be done for individual PC forms. Concluding, despite the importance of SIC and PC for terrestrial C − Various plant species such as shrubs, grasses and herbs have dif- stocks and the global C cycle, the number of studies on SIC is very limit- ferent root systems, rooting depth and resistance to higher pH ed, especially compared to those dealing with SOC. Most of these studies

due to CaCO3 accumulation. How various plant species affect PC were descriptional, focused on the presentation of properties, contents, formation rates as well as the depth of PC accumulation should forms and depths of PC. Only few studies attempted to develop the con- be clarified. cepts and models of PC formation mechanisms and relate them to envi- − Formation mechanisms of various PC features and the budget ronmental factors. Such a mechanism-based understanding and models of the elements (e.g. Ca) remain unclear. More studies such will strongly contribute to predicting terrestrial C stocks and changes in 14 K. Zamanian et al. / Earth-Science Reviews 157 (2016) 1–17 the global C cycle. This will help closely link long-term geological with Boettinger, J.L., Southard, R.J., 1991. Silica and carbonate sources for aridisols on a granitic pediment, Western Mojave Desert. Soil Sci. Soc. Am. J. 55, 1057. http://dx.doi.org/10. short-term biological C cycles. 2136/sssaj1991.03615995005500040028x. Borchardt, G., Lienkaemper, J.J., 1999. Pedogenic calcite as evidence for an early Holocene Acknowledgements dry period in the San Francisco Bay area, California. Geol. Soc. Am. Bull. 111, 906–918. Braissant, O., Cailleau, G., Dupraz, C., Verrecchia, E.P., 2003. Bacterially induced mineralization of calcium carbonate in terrestrial environments: The role of exopolysaccharides and We would like to acknowledge the German Research Foundation aminoacids.J.Sediment.Res.73,485–490. http://dx.doi.org/10.1306/111302730485. (DFG) for their support (KU 1184/34-1). Special thanks to Dr. Otto Brock, A.L., Buck, B.J., 2005. A new formation process for calcic pendants from Pahranagat Ehrmann (http://www.bildarchiv-boden.de) for providing us photos Valley, Nevada, USA, and implication for dating quaternary landforms. Quat. Res. 63, 359–367. http://dx.doi.org/10.1016/j.yqres.2005.01.007. of earthworm biospherolith (Fig. 2, right) and calcite hypocoating (Fig. Brock, A.L., Buck, B.J., 2009. Polygenetic development of the Mormon Mesa, NV petrocalcic 4, right). Special thanks to Miss. Yue Sun for drawing graphics in Figs. horizons: geomorphic and paleoenvironmental interpretations. Catena 77, 65–75. 3, 4, 6 and 8. We would like to thank the Soil Science Department, Uni- http://dx.doi.org/10.1016/j.catena.2008.12.008. Bughio, M.A., Wang, P., Meng, F., Qing, C., Kuzyakov, Y., Wang, X., Junejo, S.A., 2016. versity of Tehran (Karaj, Iran), for their help in preparing soil thin sec- Neoformation of pedogenic carbonates by irrigation and fertilization and their contri- tions and the University of Tarbiat Modares (Tehran, Iran) for SEM bution to carbon sequestration in soil. Geoderma 262, 12–19. http://dx.doi.org/10. images of calcified root cells (Fig. 3). 1016/j.geoderma.2015.08.003. 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